White is fighting for an advantage now. In case of a queen exchange, his dangerous b- and c-pawns can move fast.
19... e6c4 A good decision. The knight is stronger than the bishop. 20. c5c4  20... f3f5 This is not the machine's first move, and I think it took Ding out of his preparation. 21. c4c6 Ding thought for more than 40 minutes on this move. This seems a bit too much. 21... f5e5 